Change test structure to follow closer newer Rails 5 rules

* Create new class called ApplicationIntegrationTest, so we don't have
  to override ActionDispatch::IntegrationTest
* Move UI tests to inherit from ApplicationSystemTestCase
* Existing REST API or EPP tests inherit from
  ApplicationIntegrationTest.
* Move `require 'application_system_test_case'` at the end of
  `test_helper`

I don't particularly agree with the Rails' convention of treating UI
tests as system tests and API tests as integration tests, but I see no
benefit in actively fighting against it.

test/system/admin_area/mail_templates/new_test.rb

@@ -0,0 +1,14 @@
+require 'test_helper'
+
+class AdminAreaNewMailTemplateTest < ApplicationSystemTestCase
+  setup do
+    sign_in users(:admin)
+  end
+
+  def test_new_mail_template_does_not_throw_template_error
+    visit admin_mail_templates_url
+    click_link_or_button 'New'
+    assert_text "HTML body"
+    assert_text "New mail template"
+  end
+end